In recent years, various viral and nonviral (synthetic) vector systems have been developed to introduce foreign matters such as genes into cells or living organisms. In general, for intracellular gene delivery, viral vector systems can introduce foreign matters more effectively than nonvial vector systems. However, viral vectors can have cause problems due to simultaneous expression of essential genes of the parent virus, leaky expression of viral genes, immunogenicity and modification of the host genome structure. While nonviral vectors are less cytotoxic and immunogenic, nonviral vector systems have a problem that nonviral vectors cannot introduce genes as efficiently as some viral vectors because in nonviral vector systems, foreign matters are taken up by cells endocytotically.
To overcome this problem, a liposome having the membrane fusion activity of sendai virus (hemagglutinating virus of Japan: HVJ) called a hybrid vector, i.e., a HVJ-liposome complex, was proposed. The membrane fusion activity of the virus enables intracellular gene transfer into cells and living organisms. This technique is frequently used in animal experiments worldwide (patent document 1 and non-patent documents 1 and 2). The HVJ-liposome is constructed by fusing preliminarily UV-inactivated HVJ (inactivated HVJ envelope) with a liposome loaded with a protein, a chemical substance or a gene. However, in addition of preparation of the inactivated HVJ envelope, cumbersome steps such as preparation of the liposome and isolation of the HVJ-liposome after fusion are required.
Therefore, direct encapsulation of genes into an inactivated HVJ envelope was proposed (patent document 2). In this system, the gene to be introduced into cells are encapsulated in an inactivated HVJ envelope by freeze-thawing or surfactant treatment in the presence of the gene, and the envelope is brought into contact with the recipient cells to introduce the gene through membrane fusion. This relatively easy gene transfer system using the mass-producible inactivated HVJ envelope has been used for introduction of foreign matters into cells in many studies.
On the other hand, though the inactivated HVJ envelope can be cryopreserved (−70° C.) in suspension without losing its gene transfer ability, no reports have been made about its freeze-drying storage, presumably because freeze-drying spoils the function or structure of the inactivated HVJ envelope having membrane fusion activity. However, a freeze-dried composition of the inactivated HVJ envelope which can be stored at higher temperatures has been demanded in view of transportation and storage.    Patent Document 1: U.S. Pat. No. 5,631,237    Patent Document 2: WO01/57204    Non-patent Document 1: Dzau, V. J. et al. Proc. Natl. Acad. Sci. USA, 93, 11421-11425 (1996)    Non-patent Document 2: Kaneda, Y. et al. Molecular Medicine Today, 5, 298-303 (1999)